To prepare a KBr pellet for FTIR, the process involves mixing a very small amount of the sample with KBr powder, pulverizing the mixture, drying the KBr, and then pressing the mixture into a transparent pellet under vacuum conditions. The typical ratio of KBr to sample is 100:1, and the pellet size should be between 3 mm to 13 mm, depending on the specific FTIR equipment used.
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Sample and KBr Mixture: The first step is to mix the sample with KBr powder. The sample should be about 0.1 to 1.0% by weight of the total mixture. This means for every 200 to 250 mg of KBr, only 0.2 to 2.5 mg of sample is used. This high ratio ensures that the pellet remains transparent and suitable for infrared transmission.
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Pulverization and Drying: After mixing, the KBr and sample mixture is finely pulverized to ensure a homogeneous distribution of the sample within the KBr matrix. The KBr powder itself should be pulverized to a maximum of 200 mesh. The KBr powder is then dried at approximately 110°C for two to three hours to remove any moisture. It's important to dry the KBr slowly to prevent oxidation, which can lead to discoloration.
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Degassing and Pellet Formation: The dried KBr and sample mixture is placed into a pellet-forming die. The mixture is then subjected to a vacuum of several mm Hg to remove any remaining air or moisture. Applying a vacuum helps in creating stronger, more transparent pellets that do not scatter light. A pressing load of approximately 8 tons is then applied for several minutes to form the pellet.
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Storage: After the pellet is formed, it should be stored in a desiccator to prevent moisture absorption, which could affect the quality of the FTIR spectra.
This method is specific to FTIR applications and ensures that the sample is embedded in an IR-transparent medium, allowing for accurate and sensitive spectroscopic analysis. The detailed process ensures that the pellets are of high quality, suitable for use in FTIR spectroscopy, and capable of providing reliable data.
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